Electroless plating refers to chemical deposition on a receptive surface of an adherent metal coating, for example a nickel coating, in the absence of an external electrical source. Electroless plating or deposition is also called autocatalytic plating, thereby referring to deposition in which a chemical reducing agent in solution is applied to reduce metallic ions to a metal. This metal is deposited on a suitable substrate. The plating takes place only on catalytic surfaces rather than throughout the solution. The catalyst is initially the substrate and, subsequently, the metal initially deposited on the substrate.
One apparatus for electroless plating of nickel on an alumina substrate is shown in FIG. 1. This apparatus is typically used for plating hard disk drive media. A 35-gallon stainless steel plating tank 110 with a Teflon.TM. lining is filled with a nickel plating solution 112. The stainless steel tank 110 is positioned inside a tank 114 filled with ethylene glycol solution 116. A heating element 118 is positioned inside the tank 114 within the ethylene glycol solution 116 to heat the solution 116. Heat is conducted through the ethylene glycol solution 116 to the nickel plating solution 112.
Unfortunately, the ethylene glycol does not heat evenly throughout the ethylene glycol bath. Localized heating occurs near the heating element 118 and the region of the nickel plating solution 112. For this reason, when plating is conducted in the nickel plating bath, resulting plated layers tend to be thicker near the heating element 118. This electroless plating arrangement only controls temperature of the plating solution 112 to within 3.degree. C. and local temperature variations of plating solution 112 of about this magnitude typically exist. Temperature of the plating bath in the vicinity of the heating element 118 is somewhat higher than bath temperatures removed from the heating element 118 so that the metal thickness of a substrate portion nearest the heating element 118 is significantly greater. The plating rate varies as a function of temperature so that these local variations in temperature lead to substantially varying metal layer thicknesses.
In the plating of hard disk drive media, variations in plating rate and resulting local variations in metal thickness are tolerated since the nickel metal layer is subsequently lapped or machined to a desired thickness.
Standards of electroless plating of other objects are more stringent. One example of a device having strict standards for electroless plating thickness is a thin film magnetic head gap. In magnetic recording, a magnetic media is moved at a uniform speed past poles of an electromagnet and is longitudinally magnetized. Variations in the current supplying the electromagnet produces corresponding variations in magnetization. During reproduction, the process is reversed. The magnetic media is fed past an electromagnet--a replay head--and variations in magnetization induce currents in magnetic coils corresponding to the original magnetizing currents. The electromagnet used to record, reproduce or erase the signal is called a magnetic head, or simply head. Referring to FIG. 2, there is shown an embodiment of a magnetic head 150 including magnetic pole pieces 152 and 154 wound with a coil (not shown). A separation between the pole pieces 152 and 154 is called a gap 156 with the distance between the pole pieces 152 and 154 being called a gap length. A small gap length produces a sharp record and, therefore, a more faithful reproduction. A thin film magnetic head 150 is formed by electroless plating of a thin film on the vertical sidewalls 158 of the gap 156.
The thin film head gap 156 is plated on vertical sidewalls 158 rather than deposited on a flat, horizontal surface. An electroless plating apparatus for plating the gaps of thin film heads must perform to very strict standards with respect to deposition thickness and several other parameters. Heights of various layers must be very precisely controlled. Failure to achieve these standards, even to a slight degree, typically results in unacceptable quality of the heads. The autocatalytic process of electroless plating, in which plating takes place on the catalytic surface of deposited metal, is very sensitive to variations in temperature in the plating bath. These conditions produce an unsuitable metal film layer with a film of nonuniform thickness.
The described electroless plating system is unsuitable for plating thin metals to rigorous standards of thickness uniformity required for fabrication of a thin film head gap.
What is needed is an apparatus and method for plating a thin-film head gap which provides a precisely uniform temperature throughout the plating bath and avoids localized heating within the bath.